The present invention relates to measurements of partially light-transmissive thin films or layers of such films, and to measurements of surface topography or detection of surface defects. It specifically relates to optical measurement or detection, and to apparatuses for performing such optical measurement or detection of a thin film or a substrate surface.
A number of common articles of manufacture now have constructions involving thin films formed on relatively large area smooth substrates, and substrates wherein the underlying surface is reflective, possibly conductive, and at least visually smooth if not optically flat. To develop manufacturing processes for reliably fabricating these articles and to inspect them or understand the defects which arise in these articles, it is necessary to observe the thin films and the underlying substrate. These films may be liquid or solid, have a thickness substantially under one wavelength of the observation illumination, and may possess features or defects which are observable only with meticulous methodology against the highly reflective substrate, requiring a special instrument. To detect changes occurring on such a thin surface coating is an even more challenging task.
Various optical diagnostic methods, such as reflection ellipsometry, have been proposed to study thin film layers and surfaces. Reflection ellipsometry is the measurement of the change in polarization of light upon reflection from a specular surface to obtain information about the surface. Conventional automatic ellipsometers employ a rotating optical element, usually a rotating analyzer, to measure the polarization of the specularly reflected light beam. A significant drawback of these ellipsometers is that the instruments are relatively slow and thus are not suitable for real-time analysis.
A somewhat faster ellipsometer, a polarization-modulated ellipsometer (PME), is described in a paper of Jellison and Modine (Applied Optics, Vol. 29, No. 7, pg. 959 (March 1990)). This ellipsometer employs a photo-elastic modulator that dynamically elliptically polarizes the light incident on the sample surface and separates the analyzed light into orthogonally polarized beams using a Wollaston prism. The time resolution of this system is limited by the modulation frequency of the phase modulator which is approximately 50 kHz. The optimal time resolution of this type of ellipsometer is described as 10-ms, which remains impractical for real-time or in-situ analysis during processing or, in the case of magnetic storage disks, during use.
As the above described and other prior art devices and methods for performing optical measurement or detection of a thin film or a substrate surface have proven less than optimal, it is an object of the present invention to provide nondestructive diagnostic systems and methods having improved sensitivity, speed, and time resolution.
Another object of the present invention is to provide optical measurement systems and methods in which the surface of a substrate can be analyzed by a single optical scan of the substrate surface.
A further object of the present invention is to provide optical measurement systems and methods for real-time and in situ measurement and detection of changes or defects in a thin film layer and the underlying substrate surface.
Other general and more specific objects of this invention will in part be obvious and will in part be evident from the drawings and the description which follow.
The present invention is directed to an optical measurement system for evaluating the surface of a substrate or the thickness and optical characteristics of a thin film layer overlying the substrate. The optical measurement system includes a light source for generating a light beam, a static polarizing element for polarizing the light beam emanating from the light source, and a measurement system for measuring the light after interaction with the substrate. The measurement system includes a static beam splitting element for splitting the light after interaction with the substrate into s-polarized light and p-polarized light. The measurement system further includes two optical sensors for separately measuring the amplitude of the s-polarized light and the intensity of the p-polarized light. A control system analyzes the measured amplitude of the s-polarized light and the p-polarized to determine changes in the topography of substrate or changes in the thickness or optical characteristics of the thin film layer.
A significant advantage of the optical measurement systems of the present invention is that the amplitude of s-polar and p-polar light components can be measured simultaneously, thereby increasing the speed and time resolution of the system by requiring only a single scan of the substrate to analyze the substrate. In one embodiment, the measurement system is configured to measure the light from the substrate at frequencies greater than 1 kHz. In preferred embodiments of the inventions the speed of the system can be improved to 10 MHz.
Moreover, the optical measurement system of the present invention uses static polarization, i.e. the polarization of the light incident on the substrate is not varied during measurement, thus, the speed of the system is not limited by the rotation or modulation frequency of the optical elements of the system.
The static polarizing element can be a retarder for statically elliptically, circularly, or linearly polarizing the light beam from the light source. The retarder can be, for example, a quarter-wave plate or a half-wave plate. In the alternative, the retarder can be a liquid crystal variable retarder (LCVR).
In a preferred embodiment of the invention, the optical measurement system can include a system for collecting and measuring scattered light reflected from the substrate surface to obtain information concerning the roughness of the substrate surface. The system for collecting and measuring scattered light can include an integrating sphere for collecting the scattered light and a photo-diode for measuring the intensity of the scattered light.
In one embodiment, the system includes a light source feedback system for controlling and stabilizing the light beam from the light source. The light source feedback system can include a photo-diode for measuring the intensity of the light beam and a light source controller for controlling and stabilizing the light beam based on the measured intensity. A non-polarizing beam splitter can be used to direct a portion of the light beam from the light source to the photo-diode for measurement. The light source feedback system can be integrated into the light source or, in the alternative, can be a separate, stand-alone sub-system of the illumination system of the optical measurement system of the present invention. Alternatively, the light source feedback system can be used solely to monitor the light beam from the light source, without control or stabilization of the light beam.
The optical system of the present invention preferably includes a controllable translatable assembly for moving the polarized light beam across a portion of the substrate. A position indicator can be employed to determine the particular locations on the substrate upon which the polarized light beam impinges. Preferably, the control systems compiles a data set, an image intensity map, correlating the measured amplitude of the s-polar and p-polar light with the particular location on the substrate upon which the light source impinges. The image intensity map can be stored in a memory storage device provided with the control system.
In one embodiment, the optical measurement system of the present invention, performs initial measurements on the substrate to generate an initial map of at least a portion of the substrate. A polarized light beam is directed to a plurality of measurement points on the substrate. The light from each measurement point on the substrate is separated into two orthogonally polarized light beams and the amplitude of each set of orthogonally polarized light beams is measured at a frequency of greater than 1 kHz. The control system compiles a data set, i.e. the initial map, by synchronizing the measured amplitude of each set of orthogonally polarized light beams with the location of each corresponding measurement point on the substrate. By comparison with a subsequent map, changes in the substrate, or a thin film layer overlying the substrate, can be resolved.
In accordance with another aspect of the present invention, the optical measurement system can provide for the measurement of at least three parameters simultaneously, thereby increasing the speed and time resolution of the system by requiring only a single scan of the substrate to analyze the substrate, while concomitantly increasing the sensitivity of the system to changes in the substrate surface or to changes in the thickness and optical characteristics of the thin film layer overlying the substrate. The measured parameters include the amplitude of the s-polarized and the p-polarized light components received from the substrate, as well as at least a third parameter, which can be, for example, the phase difference between the s-polarized and the p-polarized light components, the reflection angle of the light beam reflected from the substrate surface, or the amplitude of scattered light reflected from the substrate. Additionally, the present invention contemplates the simultaneous measurement of additional parameters, including all of the above-referenced parameters, as well as the simultaneous measurement of alternate combinations of these parameters.
According to further alternative embodiment of the present invention, the optical measurement system includes an intensity stabilized light source configured to generate a stabilized light beam, a polarizing element for polarizing the light beam emanating from the light source, and a detection system for measuring the light after interaction with the substrate. The detection system includes a polarization analyzing element for splitting the light after interaction with the substrate into s-polarized light and p-polarized light. The polarization analyzing element can be, for example, a polarizing beam splitter. The measurement system further includes two optical sensors for separately measuring the amplitude of the s-polarized light and the amplitude of the p-polarized light and a third optical sensor for measuring the phase difference between the s-polarized light and the p-polarized light. A control system is configured to analyze the measured amplitude of the s- and the p-polarized light and the phase difference to determine changes in the topography of substrate or changes in the thickness or optical characteristics of the thin film layer.
According to a further alternative embodiment of the present invention, the optical measurement system includes an intensity stabilized light source configured to generate a stabilized light beam, a polarizing element for polarizing the light beam emanating from the light source, and a detection system for measuring the light reflected from the substrate. The detection system includes a polarization analyzing elements for splitting the light after interaction with the substrate into s-polarized light and p-polarized light. The polarization analyzing element can be, for example, a polarizing beam splitter. The measurement system further includes two optical sensors for separately measuring the amplitude of the s-polarized light and the amplitude of the p-polarized light and a third optical sensor for measuring the reflection angle of the light reflected from the substrate. A control system is configured to analyze the measured amplitude of the s-polarized light and the p-polarized and the reflection angle to determine changes in the topography of substrate or changes in the thickness or optical characteristics of the thin film layer.